Liquid pump

ABSTRACT

A housing of a pump apparatus includes an undivided cavity, an inlet valve through which liquid can enter the cavity, and an outlet valve through which the liquid can exit the cavity. A primary piston is configured to reciprocate in the cavity to draw the liquid into the cavity through the inlet valve during an intake stroke of the piston and discharge the liquid out of the cavity through the outlet valve during a delivery stroke. A reservoir structure is configured to add, to the total volume of the cavity, an extra volume that is a smooth positive function of liquid pressure in the cavity.

TECHNICAL FIELD

This application relates to liquid pumps.

BACKGROUND

A liquid pump includes a piston that reciprocates in a cylindrical cavity. The piston draws liquid through an inlet valve into the cavity during an intake stroke and forces the liquid out of the cavity through an outlet valve during a delivery stroke.

SUMMARY

A housing of a pump apparatus includes an undivided cavity, an inlet valve through which liquid can enter the cavity, and an outlet valve through which the liquid can exit the cavity. A primary piston is configured to reciprocate in the cavity to draw the liquid into the cavity through the inlet valve during an intake stroke of the piston and discharge the liquid out of the cavity through the outlet valve during a delivery stroke. A reservoir structure is configured to add, to the total volume of the cavity, an extra volume that is a smooth positive function of liquid pressure in the cavity.

Preferably, the reservoir structure includes a secondary piston configured to retract by a displacement distance that is a smooth positive function of the liquid pressure. The extra volume equals the displacement distance times a cross-sectional area of the piston. The retraction of the secondary piston is along an axis that is perpendicular to the direction of the reciprocation of the primary piston.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a pressure washer that includes a pump;

FIGS. 2-4 are partially-schematic sectional views of the pump at different times during its operation;

FIG. 5 is an expanded sectional view of a portion of the pump, showing additional parts; and

FIG. 6 is a partially-schematic sectional view of another pump with parts similar to those in FIG. 2.

DESCRIPTION

The apparatus 1 shown in FIG. 1 has parts that are examples of the elements recited in the claims. The apparatus thus includes examples of how a person of ordinary skill in the art can make and use the claimed invention. It is described here to meet the requirements of enablement and best mode without imposing limitations that are not recited in the claims.

The apparatus 1 is a pressure washer. It includes a pump 10 for pumping a liquid from a supply line 12 to an outlet line 14. The supply line 12 has an inlet hose 20 connectable to a water spigot. The outlet line 14 has an outlet hose 22 connected to a spray nozzle 24. The pump 10 draws water from the inlet line 12 and forces it out the nozzle 24 in the form of a pressurized spray.

As shown in FIG. 2, the pump 10 includes a housing 30 defining a cylindrical primary chamber 32 centered on a primary axis Al. Liquid from the inlet line 12 enters the chamber 32 through an inlet check valve 34. The liquid exits the chamber 32 through an outlet check valve 36 to enter the outlet line 14.

The housing 30 has a cylindrical piston-bearing surface 40 defining an opening 42. A cylindrical primary piston 50 extends through the opening 42 into the chamber 32. The bearing surface 40 slidingly supports the piston 50 and forms with the piston 50 a liquid-tight seal that surrounds the piston 50. A coil spring 52 is wrapped about the piston 50. It is compressed between a spring-support flange 54 of the piston 50 and the housing 30 to bias the piston 50 axially outward.

An outlet shaft 60 of a motor 62 extends parallel to the primary axis Al. The shaft 60 is attached to a wobble plate 64 that has a slide surface 66 that pushes the piston 50 axially inward against the spring bias. The slide surface 66 is inclined relative to shaft 60, for the piston 50 to axially reciprocate as the wobble plate 64 rotates with the shaft 60.

The housing 30 further has a cylindrical secondary surface 70 defining a secondary chamber 72 adjoining the primary chamber 32. The secondary chamber 72 is centered on a secondary axis A2 perpendicular to the primary axis A1.

A secondary piston 80 is slidingly supported by the cylindrical secondary surface 70 and forms, with the secondary surface 70, a seal that surrounds the piston 80 continually as the piston 80 moves axially through the secondary chamber 72. A secondary spring 84 is compressed by and between the secondary piston 80 and the housing 30. The spring 84 biases the secondary piston 80 toward the primary axis A1, to a fully extended position into abutment with a shoulder 86 of the housing 30.

Liquid-filled portions of the first and second chambers 32 and 72 are parts of an undivided cavity 90 that is bounded by the housing 30 and the primary and secondary pistons 50 and 80. The cavity 90 has a total volume that varies gradually with movement of the pistons 50 and 80, being a smooth function of the pistons' displacements D₁ and D₂. The function is “smooth” in that there is no sudden increase or decrease in volume with change in D₁ and D₂.

Reciprocation of the primary piston 50 is defined by an intake stroke and a delivery stroke, while the cavity 90 is continuously filled with the liquid. Due to the density and incompressibility of the liquid in the cavity 90, movement of the secondary piston 80 is dependent on liquid pressure P_(cav) in the cavity 90 and substantially independent of the secondary piston's own inertia.

The delivery stroke starts with the primary piston 50 fully retracted as shown in FIG. 2, and cavity pressure P_(cav) equaling supply line pressure P_(in) plus inlet valve crack pressure P_(crack). The secondary piston 80 is fully extended, because the cavity pressure P_(cav) urging it to retract is too weak to overcome the spring bias pressing the secondary piston 80 against the shoulder 86.

Thereafter during the delivery stroke, the primary piston 50 moves axially inward (arrow 95) as shown in FIG. 3. When the cavity pressure P_(cav) exceeds outlet line pressure P_(out) plus crack pressure P_(crack), the outlet valve 36 opens to let the liquid into the outlet line 14.

Further extension of the primary piston 50 delivers liquid to the outlet line 14, while P_(cav) remains constant at P_(out)+P_(crack). Concurrently, the secondary piston 80 is displaced from its fully extended position by distance D₂, which is a smooth positive function of cavity pressure P_(cav) and therefore also a smooth function of the primary piston's displacement distance D₁. Extension of the primary piston 50 subtracts a displacement volume V₁, proportional to D₁, from the total cavity volume V_(cav). This is partially offset by retraction of the secondary piston 80, which adds to the total volume a displacement volume V₂ that is, advantageously, smoothly related to D₂ and equals D₂ times the cross-sectional area of the chamber 72 and piston 80. The delivery stroke ends with the primary piston 50 fully extended as shown in FIG. 4, with cavity pressure P_(cav) equaling P_(out)+P_(crack).

The intake stroke starts with the primary piston 50 fully extended as shown in FIG. 4. As the primary piston 50 retracts, spring bias gradually returns the secondary piston 80 to its fully extended position as cavity pressure P_(cav) gradually decreases. When P_(cav) reaches P_(in)+P_(crack), the inlet valve 34 opens to let liquid from the supply line 12 into the cavity 90. Further retraction of the primary piston 50 draws liquid through the inlet valve 34 into the cavity 90, while P_(cav) remains constant at P_(in)+P_(crack) and the secondary piston 80 remains fully extended. The intake stroke ends as shown in FIG. 2, with the primary piston 50 fully retracted and the secondary piston 80 fully extended.

The secondary piston 80 thus functions as a reservoir. During the delivery stroke, the secondary piston 80 accumulates a displacement volume V₂ of liquid that would otherwise be delivered to the outlet line 14, and completely returns that volume V₂ to the cavity 90 during the intake stroke. The pump's overall delivery rate, in cubic foot per minute, is thus reduced by an amount proportional to V₂, which is a smooth positive function of and preferably proportional to the secondary displacement distance D₂, which is itself a smooth positive function of outlet pressure P_(out). Therefore, the overall delivery rate is a smooth inverse function of P_(out).

Power that is input from the motor 62 to drive the pump 10 is typically proportional to delivery rate times outlet pressure. Since the delivery rate of this pump 10 is configured to decrease with increasing outlet pressure, the required power will tend to vary less with P_(out) than if the second piston 80 were absent and the delivery rate were independent of P_(out).

Preferably, the secondary spring 84 is selected to yield a delivery rate that is approximately inversely proportional to P_(out), i.e., proportional to I/P_(out). That would cause the input power to be approximately invariant with P_(out), so that a motor optimized for one power level at one outlet pressure would be optimal for other pressures too. This can be achieved by replacing the secondary spring 84 with a spring structure 100 having a spring constant that increases with increasing piston displacement D₂. A step-wise-increasing spring bias can be achieved using two or more concentric springs 101, 102 and 103 of different lengths as shown in FIG. 6.

Isolating the secondary piston 72 from the motion of the primary piston 50 is facilitated by the piston axes A1 and A2 being perpendicular, and also by the primary piston 50 extending over the secondary piston 80 and its axis A2 during reciprocation. To avoid the primary piston 50 blocking the secondary chamber 72, the primary piston 50 is spaced from the cavity wall 110. The extra cavity volume due to this clearance does not decrease the achievable output pressure, because the liquid is incompressible.

FIG. 7 shows another pump 10′. It has parts that correspond to those of FIG. 2 and that function in the same way as those in FIG. 2 and are denoted with primed reference numbers matching the reference numerals of corresponding parts in FIG. 2. The pump 10 of FIG. 7 differs from the pump 10 of FIG. 2 in that its secondary chamber 72′, its secondary piston 80′ and the displacement volume it provides are all located within the primary piston 50′, and the primary and secondary pistons 50′ and 80′ reciprocate along the same axis A1′.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

1. A pump apparatus comprising: a housing having a cavity, an inlet valve through which liquid can enter the cavity, and an outlet valve through which the liquid can exit the cavity; a primary piston configured to reciprocate in the cavity to draw tie liquid into the cavity through the inlet valve during an intake stroke of the piston and discharge the liquid out of the cavity through the outlet valve during a delivery stroke of the piston; and a reservoir structure configured to add, to the total volume of the cavity, an extra volume that is a smooth positive function of liquid pressure in the cavity.
 2. The apparatus of claim 1 wherein the reservoir structure includes a secondary piston configured to retract by a displacement distance that is a smooth positive function of the liquid pressure.
 3. The apparatus of claim 2 wherein the extra volume equals the displacement distance times a cross-sectional area of the piston.
 4. The apparatus of claim 2 wherein the retraction of the secondary piston is along an axis perpendicular to the direction of the reciprocation of the primary piston.
 5. The apparatus of claim 4 wherein the axis extends through the primary piston at some point during the reciprocation.
 6. The apparatus of claim 4 wherein the primary piston extends over and beyond the secondary piston at some point during the reciprocation.
 7. The apparatus of claim 2 further comprising a spring structure against which the liquid pressure acts to retract the secondary piston, with a spring constant that increases with increasing retraction of the secondary piston.
 8. The apparatus of claim 1 wherein the reservoir structure is configured for all the liquid accumulated in the extra volume during the delivery stroke to be returned to the cavity during the following intake stroke.
 9. The apparatus of claim 1 wherein the extra volume is located in the primary piston.
 10. The apparatus of claim 1 wherein the reservoir structure is configured for the amount of the liquid discharged during the delivery stroke to be less than if the extra volume did not exist.
 11. The apparatus of claim 10 wherein the reservoir structure is configured to render a liquid delivery rate that is inversely related to output pressure of the pump.
 12. The apparatus of claim 11 wherein the delivery rate is approximately inversely proportional to the output pressure.
 13. The apparatus of claim 1 further comprising a source of the liquid connected to the inlet valve, and a liquid spray nozzle connected to the outlet valve.
 14. A pump apparatus comprising: a housing having a cavity, an inlet valve through which liquid can enter the cavity, and an outlet valve through which the liquid can exit the cavity; a primary piston configured to reciprocate in the cavity to draw the liquid into the cavity through the inlet valve and discharge the liquid out from the cavity through the outlet valve; and a cylindrical chamber defined by the housing and communicating with the cavity; a secondary piston configured to retract within the chamber in response to an increase in liquid pressure in the cavity so as to add an extra volume that, throughout the entire range of retraction of the secondary piston, is limited to the space vacated by the secondary piston as the secondary piston retracts.
 15. The apparatus of claim 14 configured for all the liquid accumulated in the extra volume during the delivery stroke to be returned to the cavity during the following intake stroke.
 16. The apparatus of claim 14 wherein the retraction of the secondary piston is along an axis perpendicular to the direction of the reciprocation of the primary piston.
 17. A pump apparatus comprising: a housing having a primary chamber, an inlet valve through which liquid can enter the primary chamber, and an outlet valve through which the liquid can exit the primary chamber; a primary piston configured to reciprocate in the primary chamber to draw the liquid into the primary chamber through the inlet valve during an intake stroke of the piston and discharge the liquid out of the primary chamber through the outlet valve during a delivery stroke of the piston; and a reservoir volume consisting of a single variable-volume space that is continuously open to the primary chamber and configured to increase in response to an increase in liquid pressure in the primary chamber.
 18. The pump of claim 17 further comprising a secondary piston configured to vary the volume of the space by retracting in response to the increase in the liquid pressure.
 19. A pump apparatus comprising: a housing having a cavity, an inlet valve through which liquid can enter the cavity, and an outlet valve through which the liquid can exit the cavity; a primary piston configured to reciprocate in the cavity to draw the liquid into the cavity through the inlet valve during an intake stroke of the piston and discharge the liquid out of the cavity through the outlet valve during a delivery stroke of the piston; and a reservoir structure configured to receive a portion of the liquid from the cavity during the delivery stroke and to return the entire received portion back to the cavity during the following intake stroke.
 20. The apparatus of claim 19 wherein the reservoir structure is configured to yield a liquid delivery rate that is inversely related to output pressure of the pump. 